Lesson 1, Topic 1
In Progress

Fertilization to Implantation

July 2, 2023

After ovulation the discharged ovum first enters the pelvic cavity and then finds its way into the uterine (fallopian) tubes. Sperm cells swim up the uterine tube toward the ovum. Look at the relationship of the ovary, the two uterine tubes, and the uterus in Figure 24-2. Recall from Chapter 23 that each uterine tube extends outward from the uterus for about 10 cm. It then ends in the pelvic cavity near the ovary, as you can see in Figure 24-2, in an opening surrounded by fringelike processes, the fimbriae.

FIGURE 24-2​Fertilization and implantation. ​At ovulation, an ovum is released from the ovary and begins its journey through the uterine tube. While in the tube, the ovum is fertilized by a sperm to form the single-celled zygote. After a few days of rapid mitotic division, a ball of cells called a morula is formed. After the morula develops into a hollow ball called a blastocyst, implantation occurs.

Sperm cells that are deposited in the vagina must enter and “swim” through the uterus and through the uterine tube to meet the ovum. Fertilization most often occurs in the outer one-third of the oviduct as shown in Figure 24-2.

The fertilized ovum, or zygote, is genetically complete—it is a new single-celled offspring. Time and nourishment are all that is needed for expression of characteristics such as sex, body build, and skin color that were determined at the time of fertilization. As you can see in the figure, the zygote immediately begins mitotic division, and in about 3 days a solid mass of cells called a morula is formed (see Figure 24-2). The cells of the morula continue to divide, and by the time the developing embryo reaches the uterus, it is a hollow ball of cells called a blastocyst.

During the 10 days from the time of fertilization to the time when the blastocyst completes implantation in the uterine lining, few nutrients from the mother are available. The rapid cell division taking place up to the blastocyst stage occurs with no significant increase in total mass compared with the zygote (Figure 24-3). One of the specializations of the ovum is its incredible store of nutrients that help support this embryonic development until implantation has occurred.

FIGURE 24-3​Early stages of development. ​A, Fertilized ovum or zygote. B to D, Early cell divisions produce more and more cells. The solid mass of cells shown in D forms the morula—an early stage in embryonic development.